1. Field of the Invention
The present invention relates to a spectroscope which is installed in an X-ray beam line using an undulator as a light source in high luminance radiant beam facilities for synchrotron, and particularly to a monochromator for radiant X-rays which is used as part of apparatus for X-ray analysis of structure and material evaluation.
2. Prior Art
According to a conventional monochromator, as shown in FIG. 14, one surface of a plate-like monochromator or a plate crystal serves as a reflecting surface, and another surface serves as a cooling surface. The reflecting surface is heated, and the cooling surface is cooled by a cooling material or a coolant such as water or liquid metal. In a certain type of monochromator, the cooling surface is finned or processed likewise so as to increase a cooling efficiency. However, in such plate type monochromators, heat applied to the reflecting surface causes stress deformation, and thus the surface of a crystal thermally deforms with a resultant problem of the scattering of emissive X-rays.
FIG. 1 shows typical thermal deformations of a monochromator crystal. As shown in FIG. 1(a), when the entire surface of the plate crystal is irradiated with incident X-rays and thus heated uniformly and also when the other surface is cooled, only a grating constant expands uniformly, while planes of atoms remain uninclined. Thus, a uniform expansion (lattice expansion) occurs in which an expansion reduces as it goes downward. In this case, a warp (bowing) (FIG. 1(c)) occurs with the heated surface being convexed. If a local zone is irradiated, only the irradiated portion swells like a knot or a bump (FIG. 1(b)). These thermal deformations of a plate crystal cause a pencil of emissive X-rays to diverge as represented with E' and E" when the surface of a crystal is irradiated with a parallel pencil of incident X-rays. That is, X-rays are reflected in nonparallel, causing a marked deterioration in spectroscopic performance (energy resolution and intensity) of X-rays.
Particularly, in designing a beam line in high-luminance radiant beam facilities most of which are occupied by an insertion light source, the aforesaid thermal deformation of a crystal is a most serious problem. Various devices have been adopted, but most of them are intended to physically improve a cooling capability. A monochromator with practical specifications has not been obtained yet. Too much emphasis is rather placed on an improvement of cooling capability, leading to a problem of instability at a lower accuracy, difficulty in use, higher costs, complicated maintenance and the like.
FIG. 2 shows an inclined crystal monochromator which has been developed for use with APS, U.S.A. (Advanced Photon Source, U.S.A.; energy 7.0 GeV; characteristic photon energy 19.0 KeV; 34 beam lines +.alpha.; circumferential length 1104 m). An angle between incident X-rays and lattice planes can never be changed, but the surface of a crystal can have an arbitrary angle. As shown in FIG. 2, (111) lattice planes (crystallographic planes related to spectroscopy) of a single crystal of silicon are laid horizontally, and the single crystal is cut at an angle of near 90 degrees from crystallographic planes to obtain an inclined surface. When X-rays impinge on the inclined surface, their shadow is cast long thereon, and thus a heat flux per unit area becomes smaller, whereby generated heat can be reduced. This means that the shadow should be cast long so as to reduce generated heat, denoting a need for a large crystal. FIG. 3 shows measurements of a ratio of an irradiated area on the surface of a crystal to the orthogonal cross-sectional area of a beam as an angle of inclination of a crystal from incident X-rays is varied. As seen from FIG. 3, as the angle of inclination increases, the ratio increases. That is, as the angle of inclination of a crystal from a pencil of incident X-rays increases, an area of shadow of incident X-rays expands on the surface of a crystal, whereby a heat flux per unit area reduces. Also, in this geometry, a direction of thermal deformation mostly falls on crystallographic planes of a crystal, thereby producing an advantage that lattice planes related to diffraction are free from large distortion. However, in spite of these advantages, the inclined crystal monochromator is said to have the following disadvantages: (1) a large crystal needs to be used so as to reduce generated heat; (2) adjustment is difficult to make (an adjustment error is greatly amplified); and (3) a fluctuation in the position of an incident beam causes instability of the position of an emissive beam. Thus, problems with respect to practical use remain to be solved.